Acoustical material

ABSTRACT

An acoustical material as a sonar reflector or decoupler is provided by forming open porous fiber webs of filament-forming thermoplastic resin, stacking the webs to form a loose pile, and pressing the pile to produce a compacted panel of acoustical material which is easily handleable and machineable. The preferred resin is polypropylene.

United States Patent 1191 Pihlstrom NOV. 20, 1973 ACOUSTICAL MATERIAL [75] Inventor: Lance W. Pihlstrom, Woodbury,

Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.

221 Filed: Mar. 5, 1971 21 Appl.No.: 121,560

52 US. Cl 161/43, 161/150, 161/157, 161/170, 156/181, 156/306, 181/.5 A, 18l/.5

ED, 181/33 E, 340/8 D 51 Int. Cl D04h-l/04 [58] Field of Search 161/150, 156, 170, 161/157, 43; 181/33 G, .5 A, .5 ED, 33 E;

[56] References Cited UNITED STATES PATENTS 3,021,914 .2/1962 Wilson 161/43 X 3,120,875 2/1964 Graner 181/.5 A

Primary ExaminerCharles E. Van Horn Attorney-Alexander, Sell, Steldt & De Lahunt [57] ABSTRACT An acoustical material as a sonar reflector or decoupler is provided by forming open porous fiber webs of filament-forming thermoplastic resin, stacking the webs to form a loose pile, and pressing the pile to produce a compacted panel of acoustical material which is easily handleable and machineable. The preferred resin is polypropylene.

5 Claims, No Drawings ACOUSTICAL MATERIAL This invention relates to sonar devices. More particularly, the invention relates to an acoustical material useful as asound wave reflector or decoupler in association with such sonar devices, and to a method of making the same.

Sonar is a term used to designate a system that employs the use of sound waves to detect the presence and location of underwater objects. The distance between the object and the source of sound waves can be determined by measuring the time it takes for sound to be sent to and reflected back from the object. The senders and receivers of the sound waves are generally transducers, i.e., devices which are capable of converting electrical energy into sound waves, and sound waves into electrical energy. Such devices are not generally constructed for unprotected use in water. Therefore, for use it is desired to encase the transducer assembly in a housing constructed of materials having specific sound transmitting qualities for controlling the intensity and direction of sound waves, and having physical properties capable of withstanding submersion in an aquatic environment, including seawater.

Acoustic materials can be divided into two general categories, i.e., that of having matched or mismatched acoustic impedance properties as compared with the environment, i.e., water in this case. The acoustic impedance of any material is represented as the product of its density and its sound transmission velocity. When the acoustic impedance is the same as that of the surrounding or adjacent material, the materials are said to be matched; when the impedance is different, the materials are said to be mismatched.

Among the matched materials are acoustic elements known as refractors, windows and absorbers. A refractor is a material through which sound waves will pass without appreciable attenuation. Refractors will have a different sound transmission velocity than that of a surrounding material. Window acoustic elements are similar to refractors in that they permit the passage of sound waves without appreciable attenuation, but the sound transmission velocity of the window will be essentially the same as that of the surrounding or adjacent material. Therefore, a window will allow the passage of sound waves with little attenuation, reflection or refraction. Absorbers are matched materials but they have a high attenuation factor, i.e.,.sound waves are appreciably attenuated or reduced in intensity as they pass through such a material.

Among the mismatched materials are acoustic elements known as decouplers and reflectors. Decouplers also have a high attenuation factor, providing maximum isolation of sound. Reflectors will dissipate a minim um amount of sound energy while reflecting or redirecting the path of the sound wave.

The present invention is primarily concerned with an acoustical material useful as a decoupler or a reflector in undersea sonar applications. It is also desired to have a material that is easy to handle and that can be formed or machined.

Heretofore the prior art materials used in such applications have beendeficient in one or more aspects. Cork particles, and various cellular or fibrous inorganic materials, held in an organic binder have been used as acoustic materials. These, however, are fragile or expensive to use. The abovementioned prior art material may also be subject to water damage because of the hydrophilic nature of some of the materials from which they are constructed.

The present invention provides an acoustic material having substantial structural integrity with acoustic and mechanical properties making such material useful for sonar use. The material has a high degree of flexural strength. The desirable strength properties allow the acoustic material to be handled, formed or machined and otherwise used without taking special precautions.

The acoustic material has an acoustic impedance value that is less than that of water therefore it is a mismatched material. As such it is useful as an acoustic reflector or a decoupler. The acoustic material has a low density, provided by a large number of minute voids therein, which contributes to the low impedance value.

The acoustic material is produced by forming open, porous, low density intertwined fiber webs of very fine fibers of a thermoplastic, synthetic polymeric resin, stacking many of the webs in a loose pile, and pressing the stacked webs at high pressures to compress and consolidate the stack to cause bonding together of the fibers until a rigid panel is formed. Although no heat or binders are used to integrate the structure, the panel has surprising structural integrity. Such integrity permits the acoustical material to be formed and handled without special precautions.

A web is formed from long, very fine synthetic organic fibers having an average diameter of about 0.1 to about 25 microns, the preferred fibers being from about 0.5 to about 6 microns in diameter. Acoustical materials may be prepared from webs of a larger diameter fiber, however, the webs of smaller diameter fibers are preferred because they are more easily formed into integrated structures. Webs having fiber diameters greater than 25 microns are not useful because they are difficult to compact and integrate unless extremely high pressures are applied. Use of such high pressure is presently impractical because of equipment limitations. Acoustical materials formed of webs having fiber diameters greater than 25 microns would also be subject to delamination caused by inadequate fusing of fibers. The smaller diameter fibers tend to fuse together more easily under pressure than thoe having a larger diameter therefore they are preferred.

The tiny fibers are preferably formed in accordance with the procedure described in Naval Research Laboratory Report No. 111,437, dated Apr. 15, 1954, entitled Manufacture of Superfine Organic Fibers." This procedure involves extruding a fine stream of molten polymeric material into a stream of heated air which causes attenuation of the extruded material into tiny fibers. Preferred polymers for forming such fibers for the acoustic materials of the invention have a density within the range of about 0.80 gm./cc. to about 1.50 gm./cc. Exemplary polymers include thermoplastic polypropylene, polyethylene, polyesters, homopolymer of 4-methyl-pentene-l polyamides such as nylon, polycarbonates, and polyphenylene oxide. Other thermoplastic polymers may also be used. The fiber diameter rather than the particular thermoplastic resin appears important in providing a useful acoustical material.

The fibers should have a minimum length of about 0.5 cm. to assist in forming a web which is sufficiently coherent to be handled. The fibers are generally found to be raveled and intertwined sufficiently to provide a web which is sufficiently dimensionally stable for handling. The preferred fiber webs are obtained by collecting the fibers on a collection surface at a distance of 7 to 8 inches (about l7 to cm.) from the nozzle orifice. Webs produced at this distance will have a lofty open nature permitting interengagement of fibers in adjacent webs which provides a more uniformly integrated product.

Closer collection points causes the individual fibers to mat into a dense paper-like web which requires considerably higher lamination pressures to produce an integrated product. The acoustical material produced from paper-like webs is more subject to delamination due to the lack of interengagement of adjacent webs.

The characteristics of the acoustical material are also affected by the thickness and density of the web. The acceptable range of web density has been found to be about 0.0] to about 0.30 grams per cubic centimeter with optimum range being about 0.02 to 0.10 grams per cubic centimeter. Webs having a density more than about 0.30 grams per cubic centimeter are undesirable because of undesirably high acoustic impedance produced in the resulting acoustic material.

The web is cut into sections that are stacked one on top of another to form a loose pile. Successive web sections may be rotated at an angle to each other to equalize density variations which may exist within the web. The stack of web sections is placed in a hydraulic press and pressed without heating under high pressure sufficient to compact and consolidate the pile. Under this pressure adjacent fibers fuse together causing the acoustic panel to achieve the aforementioned structural integrity. Pressure is maintained until such fusing and integration of the material is achieved. Generally, fusing and integration of the material will be achieved upon maintaining the pressure for about ten minutes, but this may vary for different resins and longer or shorter pressing times may be required. The panels can be made in any desired thickness, depending upon the capacity of the press, by simply varying the number of webs in the pile. The density of the acoustical material will vary depending upon the pressure applied in compacting the webs and upon the thermoplastic resin used. Such pressure will provide adequate structural integrity while still permitting the removal of individual substantially intact webs.

After pressing, the acoustical material can be cut, machined, or otherwise shaped by conventional cutting or machining tools to any desired shape. Once the panel is shaped into the desired structural configuration, it may be desired to seal or cover its outer surface, to prevent water from entering the minute pores in the panel thereby changing its acoustical properties. Sealing the surface can be accomplished by several well known procedures such as dipping the structure in a melt or solvent solution of a sealant, or by spraying the surface of the structure with such sealant. Exemplary sealants include acrylate resins, epoxide resins, polyvinylidene chloride, and neoprene rubber. The sealant can be applied at various thicknesses depending upon the desires and the needs of the user. Some change in the acoustical properties may result from such sealing depending upon the acoustic properties of the sealing material selected, therefore, care should be exercised in selecting the sealant.

The invention will be further explained with reference to the following examples in which all parts are given by weight, unless otherwise indicated.

EXAMPLES Sound transmission velocity determinations through test samples of the examples were made by placing the sample between two transducer assemblies (Probe Sonic PF-lOl-SO made by Channel Industries) with the transducers abutting the sample. The time required for a pulsed signal to pass through the sample was determined from the single image as shown in an oscilloscope (Tektronix 5 61A with 3A6 dual trace amplifier and 334 time base). The equation for calculating velcoity is as follows:

Velocity Distance/Time The percent sound transmission through test samples of the examples was determined using two Ray Jefferson Depthometer 500 Transducers to provide approximately a 200 KH resonant frequency and a 0.75 cm. wave length in water. One transducer, used as a sending transducer was submerged in a water-filled test tank with its face perpendicular to the surface and mounted therein on a vertical track. The other transducer, used as a receiving transducer, was mounted in the test tank on the track facing the sending transducer with its face parallel to and spaced therefrom 71.1 cm. Using the water as a control, a 100 percent transmission reference signal was established by adjusting the signal voltage received by the receiving transducer to full scale on the oscilloscope. A test sample was inserted between the transducers, the signal again sent, and the amount of signal received at the receiving transducer observed. This divided by the 100 percent transmission signal, multiplied by 100 gives the percent transmission.

Reflectance was determined by adjusting to full scale the signal voltage reflected back to a sending transducer (which was also a receiving transducer) from an essentially completely reflective sample. The reference reflector was then replaced by the sample to be tested and the reflectance therefrom measured. The percentage reflectance is obtained by dividing the reflectance measurement of the test sample by the reference signal and multiplying the result by 100.

The flexural strength (also called the modulus of rupture) was determined in a manner similar to that described in ASTM D 790-63 by placing a test specimen approximately l/2 inch by l/2 inch by 6 inches upon 7 two supports spaced 4 inches apart. A load was applied in the center at a crosshead speed of 0.5 inch per minute. The flexural strength is the flexural stress in pounds per square inch at 5 percent strain.

All of the abovementioned tests were carried out under ambient atmospheric conditions, e.g., 22 C., unless otherwise specified.

EXAMPLE I A web made up of-bundles of microfine polypropylene fibers was formed as follows: First a polypropylene resin having a density of 0.905 gm./cc. melting point of 333 F. (169 C.) and a nominal melt flow rate of 12 gm./l0 min. was extruded through 2 dies spaced about 6 inches apart and at a slight angle toward each other. Each die had 250 0.014 inch (0.035 cm.) diameter orifices arranged in a line. The extruder was operated at a temperature of 700 F. (371 C.), the extrusion die temperature being 625 F'. (330 C.). The extruder was operated at a rate of 14 lbs. (6.35 kg.) of resin per hour. The resin emerging from the die was immediately blasted with hot air at 700 F. (370 C.), which was dis charged frm a 3/4 inch (1.9 cm.) opening at a pressure of 5 psil The web was collected at a rate of 21 ft. per minute (6.4 meters per minute) at a distance of about 7 inches (18 cm.) from the extrusion die. A fibrous web about 48 inch (122 cm.) in width and having a void volume of 94 percent was formed consisting of loosely raveled tiny individual fibers having diameters of 0.5 to 3 microns with an average diameter of about 1.5 microns, and a virtually continuous length.

The fibrous web was cut into 50 12 inch square sections which were stacked one on top the other, rotating each successive sheet 90 to equalize density variations which may have existed in the web. The 50 sheets produced a loose pile about 12 inches in height. The pile was placed between flat pressing surfaces of a hydraulic press and compressed therein at a bulk pressure of 188,000 pounds 1300 psi) to produce a l inch (3.5 cm.) thick panel. The pressure was maintained for minutes to assure complete fusion of fibers and compaction of the acoustical material. The resultant laminate had a measured density of 0.46 grams per cubic centimeter and a void volume of 49 percent. The panel was subjected to the aforementioned velocity, reflection, and transmission tests with results as follows:

Velocity-700 Meters per second Reflection-90 percent Transmission-6 percent EXAMPLES 2-6 In a manner similar to that described in Example 1, the following examples were prepared by pressing polypropylene fiber webs as described in Example 1 to produce acoustical materials as follows:

Examples 2-6 were subjected to the aforementioned velocity, transmission, and reflectance measurements with results as follows:

Velocity Transmission Reflectance Example (m./sec.) 2 5 70 2 75 3 750 8 65 4 660 12 so 5 800 I6 40 6 900 so so EXAMPLE 7 An acoustic panel as described in Example 1 was spray coated with a neoprene latex solution to provide upon drying a l/16 inch water impervious coating. The acoustic properties of the coated panel were not tested.

EXAMPLE 8 An acoustic panel as described in Example 1 was heat sealed in a 5 mil heat scalable film consisting of a laminated construction of a polyethylene layer and a polyester layer to provide a coated panel. The coating did not change the acoustic properties from the values set forth in Example 1.

What is claimed is:

1. An acoustical material in rigid block form and having acoustic impedance less than that of water, thus having particular utility as a sonar decoupler or reflector, said material consisting essentially of a unitary structure of compacted integrated fibers of filament forming thermoplastic organic resin fused at loci of contact, said material having a flexural strength of at least pounds per square inch, a uniform density about 0.3 to 0.8 times the density of said resin, and parallel planes of weakness, said block being of sufficient thickness to provide sound wave decoupling or reflection in water and being characterized by the ability to be separated into distinct layers along said planes, said fibers having a diameter within a range of 0.2 to 25 microns.

2. The acoustical material of claim 1 wherein the density of said acoustical material is from 0.4 to 0.7 times the density of said resin.

3. The acoustical material of claim 1 wherein the thermoplastic organic resin is polypropylene.

4. The acoustical material of claim 1 wherein the thermoplastic organic resin is the homopolymer of 4- methylpentene l.

5. The acoustical material of claim 1 including a close fitting flexible water impervious covering thereon. 

2. The acoustical material of claim 1 wherein the density of said acoustical material is from 0.4 to 0.7 times the density of said resin.
 3. The acoustical material of claim 1 wherein the thermoplastic organic resin is polypropylene.
 4. The acoustical material of claim 1 wherein the thermoplastic organic resin is the homopolymer of 4-methylpentene
 1. 5. The acoustical material of claim 1 including a close fitting flexible water impervious covering thereon. 